1. Field of the Invention
The present invention relates to a linear motion guide unit with lubricating plate assemblies, which is applicable to industrial robots, semiconductor manufacturing apparatus, precision instruments, machine tools and the like.
2. Description of the Prior Art
The linear motion guide units have conventionally used incorporated in the parts or components of the industrial robots, semiconductor manufacturing machines or the like, where the precise control is required for reciprocating motion. The recently remarkable development in mechatronics technology extensively requires linear motion guide units. In addition, the linear motion guide units recently become required to meet with needs of increasingly severe operating conditions. For example, the linear motion guide units to be incorporated in the industrial robots for assembly plants, whether the robots are large or miniature in size for wide applications, become required to meet with needs of maintenance-free, along with high precision and high speed in operation. To cope with the needs, the linear motion guide unit has required the self-lubrication of long service life on its relatively movable sliding areas.
A linear motion guide unit as shown in FIG. 15 is conventionally well known and, for example, disclosed in Japanese Patent Publication No. 61046/1991. The prior linear motion guide unit is what is called a ball-spline bearing and composed of a track shaft 2 of a substantially circular cross section having two raceway grooves 3 that extend axially on the peripheral surface of the track shaft, and a cylindrical sliding element, or a slider 1, mounted on the track shaft 2 for sliding movement. The slider 1 may move on the track shaft 2 by virtue of rolling elements 7, or balls, running through the raceway grooves 3 on the track shaft 2 in an endless-circulating manner. The slider 1 includes a casing 5 movable along the track shaft 2 and end caps 6 attached to the lengthwise opposing ends of the casing 5, each to each ends. The casing 5 is of a hollow cylinder having therein raceway grooves 4 confronting the raceway grooves 3 on the track shaft 2, and return passageways 10 for rolling elements 7. A key way 14 is formed on the outer surface of the casing 5 for mounting other appliances, components or parts, chucks and grasping jaws or the like on the slider 1.
The end caps 6 are each provided therein with claws for scooping the rolling elements 7 out of the load raceways defined between the confronting raceway grooves 3, 4 and turnarounds to turn the rolling elements 7 for endless circulation. Mounted on the end caps 6 are end seals 8 for closing clearances between the track shaft 2 and the lengthwise opposing ends of the slider 1 thereby protecting the working area in the slider 1 from dust and dirt. The end seals 8 are usually made of core metals and rubber members of, for example, acrylonitrile-butadiene rubbers. The end caps 6 are fixed together with the end seals 8 to the opposing ends of the casing 5 by fixing bolts or screws 13 fitted in matching holes.
The load raceways defined by the confronting raceway grooves 3, 4, non-loaded turnarounds formed in the end caps 6 and non-loaded return passageways 10 formed in parallel with the raceway grooves 4 in the casing 5, in combination, constitute endless-circulating paths for the rolling elements 7. That is to say, in practice, the balls 7 may run from the raceways into any one of the lengthwise opposing turnarounds, then through the return passageways 10 and the other of the turnarounds, and circulate to the raceways. Thus, the rolling-contact of the balls 7 with the raceways may help ensure the smooth movement of the slider 1 relative of the track shaft 2.
To lubricate the raceways and the rolling elements 7 in the linear motion guide unit constructed as described above, grease or lubricating oil is usually employed. In case of grease, it is applied to the raceways through grease nipples 40 mounted to the end caps 6. In contrast, lubricating oil is supplied to the raceways through pipe joints for the lubricating oil, which are used substituting for the grease nipples.
Nevertheless, as the linear motion guide units becomes common universally with wide applications in various fields, they are increasingly used under unfavorable operating conditions or adverse environments, for example, where no replenishment of lubricating oil is allowed, much dirt and debris may occur and there is abnormally high in temperature or moisture. In recent years much attention has been thus given the development of the linear motion guide units having no fear of lubrication failures even under the severe operating conditions, where it is very hard to achieve the normal lubrication. Disclosed in, for example, Japanese Utility Model Laid-Open No. 4593/1995 is a ball nut and screw assembly including lubricant-containing polymer to ensure the satisfactory operation even under the unfavorable environment where it might be plagued with the difficulties of lubrication.
The prior ball nut and screw assembly is composed of a screw shaft having helical external grooves around the outer periphery thereof, a nut fitted loosely over the screw shaft and provided on the inner surface thereof with helical internal grooves confronting the grooves on the screw shaft, balls running through helical spaces between the confronting helical grooves, and lubricant-containing polymer members arranged so as to make slide-contact with the helical groove surfaces on the screw shaft.
The lubricant-containing polymer members in the prior ball nut and screw assembly are, however, produced by mixing poly(.alpha.-olefinic) polymer with a lubricating oil, then melting the mixture with heating in a predetermined mold, and solidifying the molten mixture by cooling. Therefore, the lubricant-containing polymer members have necessarily required the complicated and high technology as well as the high production cost. In addition, it will be understood to those skilled in the art that the technology as to the ball nut and screw assembly may not be directly applied to the linear motion guide units without any modification. It is thus expected to develop an improved lubricating means for the linear motion guide unit what is referred to as a ball-spline, which makes it possible to keep self-lubrication even under too harsh situation for lubrication.